DNA Study Reveals Secrets Of Gentically Similar Polar And Brown Bears

A new report in PLoS Genetics attempts to clarify conflicting studies on a population of Alaskan brown bears that are genetically similar to polar bears, yet look and act like typical brown bears.

“This population of brown bears stood out as being really weird genetically, and there’s been a long controversy about their relationship to polar bears,” said co-author Beth Shapiro, a professor of ecology and evolutionary biology at the University of California, Santa Cruz (UCSC). “We can now explain it, and instead of the convoluted history some have proposed, it’s a very simple story.”

Shapiro and her team believe that as the last Ice Age ended and the glaciers receded, a group of polar bears was stranded on Alaska’s Admiralty, Baranof, and Chicagof Islands, collectively known as the ABC Islands. Eventually, male brown bears would swim across to the islands from the mainland and mate with female polar bears living there, transforming the entire ABC Islands´ population into brown bears.

Previous studies on both of these bear species, which have been known to mate and known to produce fertile hybrids, suggested that all polar bears had descended from the ABC brown bears — essentially telling Shapiro´s story in reverse. However, the findings from UC Santa Cruz biologists show that the genetic exchange occurred only in isolated populations and did not affect the larger polar bear population.

Shapiro said previous research had focused primarily on the bears´ mitochondrial DNA, which is inherited directly from the mother´s DNA.

“The key to solving this mystery was to analyze DNA from the ABC Islands bears’ nuclear genomes, and in particular their X-chromosomes,” Shapiro said in a statement. “Focusing on the X gave us a surprising result.”

In the latest study, the team, which included scientists from California, Canada and Russia, analyzed DNA from seven polar bears, an ABC Islands brown bear, a mainland brown bear, and a black bear. The researchers also considered data from recently published bear studies by other researchers.

The team found that polar bears are genetically homogeneous, with no evidence of brown bear ancestry, yet the ABC Islands brown bear DNA contained clear evidence of polar bear ancestry. According to Shapiro, evidence from the bears´ X chromosome indicated that ABC brown bears have more DNA in common with polar bear females than they do with polar bear males.

To better understand the scenario behind these genetic results, the team used computer simulations of various breeding scenarios.

“Of all the models we tested, the best supported was the scenario in which male brown bears wandered onto the islands and gradually transformed the population from polar bears into brown bears,” said co-author James Cahill, a graduate student at UC Santa Cruz.

The researchers warned that the isolation of the polar bear is occurring today as a result of climate change and said there could be far-reaching genetic impacts as a result.

“During a previous ice age, polar bears ranged much farther to the south than they do today, reaching the present-day ABC Islands and Alexander Archipelago,” said co-author Ian Stirling, from the University of Alberta in Canada. “As the climate warmed and ice began to retreat, it is possible that some of the animals began to spend progressively longer on land with reduced access to ice.”

“We see the same sort of thing happening today with polar bears in areas such as western Hudson Bay or the Russian coast, in response to continued climate warming and loss of ice,” he added.